In continuous ink jet printing, ink is supplied under pressure to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s). The ink is expelled from the orifices in jets which break up due to surface tension in the ink into droplet streams. Ink jet printing is accomplished with these droplet streams by selectively charging and deflecting some droplets from their normal trajectories. The deflected or undeflected droplets are caught and re-circulated and the others are allowed to impinge on a printing surface.
To selectively charge the ink droplets, it is desirable to stimulate the ink jets to accurately control the locations that the droplets separate from the ink jets downstream from the orifice plate. Such stimulation is provided by applying a vibration to the ink, for example, by vibrating the orifice plate. Stimulation also maintains uniform drop size and drop spacing as well as controlling the location of the drop separation. It is also desirable that the droplets from all of the jets separate at the same time from their respective jets, an occurrence known as synchronous stimulation. Such synchronous stimulation simplifies the problem of drop charging, since each drop in the jet separates from the jet at a precisely predictable time period, allowing accurate drop charging and placement and avoiding printing errors due to improper droplet charging.
One problem which occurs on all droplet generators is that the motion and hence the stimulation at the boundary defined by the end of the droplet generator fluid cavity trench and the end of the orifice plate array is not the same as in the middle of the droplet generator. This leads to nonuniform break off of the jets near the ends of the array.
One known method of increasing the uniformity of break off of the jets along the entire length of the droplet generator is described in U.S. Pat. No. 4,999,647. When the orifice array is made longer than, for example 10 cm, the printhead has many other modes near 50 kHz which must be suppressed for proper operation of the printhead. The '647 patent discloses an ink jet printhead having a series of slots through the printhead body to divide the body into a plurality of approximately identical dilatational regions. These slots have the effect of decreasing the mode coupling between the desired vibrational mode necessary for synchronous stimulation and undesired modes that decrease efficiency and frustrate synchronous stimulation. As printing speeds are increased, it becomes desirable to stimulate the ink jets at increasingly higher frequencies.
It has been found, however, that print heads of the type shown in the '647 patent cannot be synchronously stimulated much above 100 kHz before mode coupling again becomes a serious problem. At such high frequencies, mode coupling occurs not only in the print head itself, but also in the piezoelectric transducers employed to drive the print head.
An alternate drop generator design for long array, high frequency applications is described in commonly assigned, co-pending U.S. patent application Ser. No. 09/211,059, totally incorporated herein by reference. The drop generator described therein utilized multi-lobed radial vibrational modes to stimulation the array of jets. It was recognized in that application that the vibration of the ends of the drop generator tends to be different than in the middle of the array. By making cuts into the top of the drop generator parallel to its long axis, the ends of the drop generator can be made to vibrate more like the center of the drop generator. It has been found however that while the cut parallel to the array on the top of the drop generator improved stimulation uniformity, that further improvements in stimulation uniformity are desirable.
It would be desirable, therefore, to improve non-uniformity of jet stimulation at the end area regions of the orifice plate array, particularly when operating at high frequencies.